1. Field of the Invention
The invention relates generally to finishing procedures for metal components and more particularly to an accelerated finishing procedure capable of producing an extremely smooth surface finish in a reduced time.
2. Description of the Related Art
Procedures for producing a smooth surface finish on a metallic component are generally well known. Such procedures include barrel tumbling, abrasive vibratory finishing, grinding, honing, abrasive machining and lapping. Examples of mechanical parts that may be finished using these procedures include splines, crankshafts, camshafts, bearings, gears, constant velocity (CV) joints, couplings, and journals. Various advantages may be achieved by such finishing including a reduction in wear, friction, noise, vibration, contact fatigue, bending fatigue and operating temperature in the mechanism to which they relate. Although not all of the mechanisms are understood by which this may be achieved, it is believed that the reduction of surface asperities and distressed metal can reduce friction and prevent scuffing, abrasive wear, adhesive wear, brinnelling, fretting and contact fatigue and/or bending fatigue at the relevant metal-to-metal contact or non-contact dynamically stressed surfaces. Alternatively, objects may be provided with a finish for aesthetic reasons or for corrosion resistance reasons. The actual effectiveness of the finish in achieving these effects appears to depend not only on the final smoothness but also on the manner in which it is achieved.
The type of finishing process is believed to play a role due to the microscopic relief that characterizes the manner in which the finish has been achieved. This can depend on the polishing mechanism, chemicals used, local temperature effects, isotropic or non-isotropic nature and many other factors.
Early vibratory finishing techniques used motor-driven vibratory bowls or tubs in which the component would be free floated and allowed to agitate in the presence of abrasive media. By free floated, it is meant the components are allowed to be carried around the vessel by the movement of the media mass. The degree and rate of finishing is primarily controlled by the coarseness, amount and or replenishment of the abrasive grit used in the media mass. Such processes are based on the mass finishing techniques used, for example, for polishing stainless steel tool handles in which ever finer polishing media is used to achieve the desired degree of finish. However, metal components such as gears or bearings found in the aerospace or automotive sectors are typically induction hardened, case carburized or through hardened to a hardness of 50 HRC or above. Conventional abrasive techniques may require unacceptably long processing times of 12 hours or more to achieve the desired smoothness. In other processes, appropriate chemicals have been introduced into the mass finishing container in order to enhance the finishing capability and action of the media. U.S. Pat. Nos. 3,516,203 and 3,566,552 are examples of such procedures. According to U.S. Pat. No. 6,261,154 to McEneny, the contents of which are incorporated herein by reference in their entirety, additional forces may be induced by rotating a workpiece around its axis in a fixed position against the flow of finishing media.
Further procedures have been developed in which increased levels of mechanical energy are imparted onto the component by moving the component through relatively stationary media. One such procedure is known as drag finishing and is described in e.g. U.S. Pat. No. 4,446,656 to Kobayashi, the contents of which are incorporated herein by reference in their entirety. According to such procedures, finishing is solely an abrasive process. The high levels of energy and the speed of abrasion can however be detrimental to the geometrical tolerance of metal components such as gears or bearings. This is particularly the case where the direction and location of media impingement on the component is not uniform over the treated surface. In an effort to improve uniformity, complex movement geometries are imparted onto the components involving rotation around multiple axes. One such drag finishing machine is described in U.S. Pat. No. 6,918,818 to Böhm, the contents of which are incorporated herein by reference in their entirety. In this device, individual components may be fixtured to a drive spindle for finishing. The total throughput of components is determined by the process time and the fixturing time for connecting and disconnecting components from the drag spindle.
One procedure that can achieve an ultra-smooth superfinished surface is chemically accelerated vibratory finishing (CAVF). A chemically accelerated vibratory finishing technique has been developed and described in numerous publications by REM Chemicals, Inc. This technique may be used to refine metal parts to a smooth and shiny surface and has been used commercially for many years. U.S. Pat. No. 4,818,333 to Michaud and U.S. Pat. No. 7,005,080 to Holland, the contents of which are incorporated herein by reference in their entirety, disclose this improved finishing technique. A significant difference between this technique and abrasive media based processes is that in a chemically accelerated finishing process, the media does not significantly abrade the metal surface. The combination of the media plus the mechanical energy imparted by the mass finishing equipment in question is not capable of effectively removing material from the surface of the component without accelerated chemistry. Mixed processes have also been suggested.
Another important characteristic of surfaces produced by CAVF is that they are planarized. This means that the uneven surface prior to finishing is made smoother by removal of the upwardly protruding asperities with little change to the form of any depressions or valleys. While not wishing to be bound by theory, the resulting surface is characterized by flat plateaus, understood to have good load bearing characteristics separated by crevices facilitating oil retention. These planarized surfaces are also believed to have the advantage of substantially no peaks that would otherwise penetrate through a lubricant film and cause damage with a mating surface. A chemically accelerated vibratory finish of below 0.5 microns Ra tends to exhibit some or all of the performance benefits discussed above.
A significant factor in the use of CAVF is the amount and concentration of chemistry used. The chemicals are acidic and excess chemistry and or concentration or elevated temperatures can cause etching of the surface of the component being finished and/or can cause other metallurgical deterioration of the metal. Components of high hardness are also often more susceptible to chemical attack such as etching from chemicals typically used in CAVF. In general, if etching occurs, components such as gears or bearings will likely be scrapped. In order to avoid such damage, the amount and type of chemistry and temperature of the process is carefully matched to the amount of media and the surface area of the components to be finished. Typically, flow-through processing is utilized. In flow-through processing, the vibratory vessel operates in an open air environment at room temperature, and is provided with a chemistry delivery system in which the accelerating liquid chemistry, at ambient room temperature, is metered continuously into the vessel during the surface refining process. Simultaneously, an open drain in a low point in the vessel continuously drains away excess liquid such that puddling does not occur during operation. To avoid etching and to operate efficiently, the amount of flow-through chemistry should be just sufficient to wet all of the media and components, and should be at a concentration just sufficient to react with the amount of surface area of the metal components being finished. Thus, excessive inflow of liquid is avoided to prevent a build up of liquid volume within the vessel to avoid etching. Similarly a blockage of the drain causing an accumulation of chemistry in the vibratory vessel can lead to etching and subsequent scrapping of all the components. Temperatures above ambient room temperature within the vibratory vessel, irrespective of the amount of liquid within the vessel, can also increase the potential of etching and scrapping of such components.
Tests have been performed in order to determine optimum conditions for CAVF. In a paper at the Tri-Services Corrosion Conference 2007 by Juergen Fischer entitled “Basic Studies Concerning Chemically Accelerated Vibratory Surface Finishing” it is concluded that greater finishing speed may be achieved using reduced chemical hold up, and that the process showed no visible temperature dependency in the range studied.
An advantage of vibratory finishing in a bowl or tub is that many individual components may be finished in a single batch. Such batch finishing may not, however, be convenient in an item-by-item (just in time) production line environment or where components must be individually identified or matched. In particular in relation to gear assemblies, it is often the case that two or more components are matched, for example, by a lapping process. Thereafter, it is desirable that the matched parts are kept together during subsequent operations. For such components, batch (mass) finishing is generally not suitable. Mass finishing may also be unsuitable in cases where delicate components may not knock against one another in the vibratory process. Many other finishing processes have been suggested and developed, but none has proven suitable for high-throughput, in-line, mass-finishing (such as a vibratory bowl, tub or tumbling barrel) of large numbers of components requiring special handling.
Thus, there is a particular need for a device and procedure that allows at least some of these problems to be overcome.